The invention relates to a passive electromagnetic wave duplexer.
In a radar system, it is absolutely necessary to protect the radar receiver both against the high level energy emitted by the transmitter and against the energy originating from neighbouring radar transmitters. However, it is highly desirable that the entire energy picked up by the antenna and coming from a target illuminated by the transmitted radiation should be transferred to the receiver without loss. The duplexer thus acts as a switch isolating the receiver during transmission or during a powerful nearby transmission and unblocking the receiver channel during reception of weak signals by the antenna.
There are at present different kinds of duplexers which will be described in the following, but which have the disadvantage that they cannot function or only function poorly with waves in the millimetric ranges. A first kind of duplexer illustrated in FIG. 1 comprises two identical limiters 1 inserted between two 3 dB couplers 51,52, each limiter 1 being unblocked for weak signals but reflective for high power signals. The first coupler 51 is connected to its input side to the transmitter 3 on the one hand and to the antenna 4 on the other hand, whereas its outputs each lead to one of the two limiters 1. The latter are connected to the inputs of the second coupler 52, whose outputs are connected respectively to the radar receiver 5 and to a dissipator load 6. During emission of the radar signal by the transmitter 3, the limiters 1 reflect the same towards the antenna 4 whereas they allow free passage to the weak signals received during reception.
A second kind of duplexer (FIGS. 2a and 2b) comprising a non-reciprocal ferrite device, operates on the following principle: The signal coming from the transmitter is directed to the antenna and any signal received by the antenna is necessarily channelled to the receiver, notwithstanding its power. FIG. 2a shows a duplexer of this nature, comprising two differential dephasing devices 74 and 75, of which the operation is the following: For an initial signal coming from the transmitter 7 and passing through a coupler 71 producing a phase difference .pi./2 between the channels 72 and 73, the differential dephasing ferrite device 74 phase shifts the signal of channel 72 by .pi./2+.rho.o whereas the other ferrite device 75 phase shifts the signal of channel 73 by .rho.o. The two signals of which the corresponding phase shifts are .rho.o+.pi./2 and .rho.o lead to a magic T 76 at whose output they are in phase again and are fed to the antenna channel 77. If a signal coming from the antenna 77 is now considered, irrespective of its power, it is dephased by .rho.o by the ferrite device 74 and by .rho.o+.pi./2 by the ferrite device 75, so that the signals respectively emerging from the devices 74 and 75 are in phase again in the receiver after passing through the coupler 71.
As for FIG. 2b, it shows a duplexer in which the non-reciprocal device is a three-channel circulator 8. To provide protection for the radar receiver against the high power transmissions of nearby radar transmitters arriving through the antenna, a supplementary limiter cell is added to this kind of duplexer in the reception channel, this cell being formed either by a TR gas tube having a comparatively short life or by ferrite or diode devices.
As stated earlier, these duplexers do not operate satisfactorily with millimetric waves, since the limiter cells described either do not exist for such waves, which is so in the case of TR tubes, or cannot stand up to power satisfactorily, which is the case for existing diodes installed in conventional structures.